THE PRODUCTION OF SEEDS

Producing Seed of Hybrid Corn and Grain Sorghum

JOHN M. AIRY, L. A. TATUM, AND J. W. SORENSON, JR.

AFTER 26 years or so of research, the first commercial hybrid corn seed for sale to farmers in any quantity was produced in Iowa in 1926.

Hybrid corn was planted on about 1 percent of the corn acreage in the Corn Belt in 1933 and on almost 100 percent in 1955. The wide adoption of high-yielding hybrid strains in 1938-1945 led to an increase of 15 to 20 percent in the average yield of corn in the United States.

The development of hybrids of grain sorghum followed discoveries of male-sterile characteristics in 1935 and later. Farmers accepted grain sorghum hybrids more quickly than they did corn hybrids. The first commercial seed field was planted in 1955. Nearly 70 percent of the acreage in 1960 was planted to sorghum hybrids.

The use of hybrid seed requires the production of new seed each year; the use of seed from farmers' fields of a hybrid would result in a loss of 15 to 20 percent in yield in the succeeding crop because of the reduction of hybrid vigor from inbreeding. An extensive business has been developed to supply the need.

Farmers need 10 million to 12 million bushels of seed corn and 1.5 million to 2 million bushels of sorghum seed each year. Large amounts of capital, labor, and technical knowledge are required of the specialized seed producers, which may be small farm-type operations or corporations. Some of the corporations conduct research on methods, develop inbred lines, produce and test hybrids, produce seed in their own plants, and sell directly to farmers. Much of the hybrid corn seed is produced by such firms.

CULTURAL practices for growing seed of corn and sorghum are similar in many respects.

Fields for seed are in high-yield areas that have fertile soil and favorable conditions of temperature and moisture.

Some corn seed is produced in irrigated areas, but much is produced in parts of the Corn Belt where normal rainfall is adequate.

Small amounts of grain sorghum seed are produced in humid areas, but more is produced west of the Corn Belt in irrigated areas where grain sorghum is a regular crop.

Before we had hybrids, seed usually was grown in areas where the variety was adapted. Seed fields for producing hybrid seed may be grown elsewhere, however, and the seed will be as well adapted as if grown locally. This is possible because hybrids are crosses of specific and uniform inbred lines, which change little or not at all when they are grown in different geographic areas or a different climate.

Potential yield in seed fields, weather risks (such as drought, high temperatures, hot winds, and hail), length of season, maturity of the hybrid parents, date of freezing temperatures, isolation from undesirable varieties, freight costs to the planned market, and other economic factors influence the decisions as to where to locate seed fields.

Seed of hybrids for the South, however, is grown in the South, and seed for the North usually is grown in the Corn Belt. Seed of corn hybrids for farthest northern areas often is grown 200 to 400 miles south of the area where it is best adapted to reduce risk of a freeze before harvest and to increase the size of the seed.

Regional limitations from south to north are not so great for most grain sorghum hybrids as for corn. Seed intended for Nebraska thus can be grown in Texas, or much of the seed for Texas can be grown in Nebraska.

Much of the seed is produced by farmers under contract with seed companies. The seedsmen pay certain costs, such as for the parent seedstock furnished to the growers and for the detasseling operation. High yields are thus mutually advantageous, and improved practices are applied to a greater degree than on the average farm. Some seed companies employ specialists who advise the growers on cultural practices.

It is important to get a stand of the correct number of plants to the acre. Heavy stands affect the size of the seeds and also the yield if drought and heat ensue.

An adequate supply of each fertilizer element is important for uniformity throughout the field, high-quality grain, early maturity, and the most profitable yields. Growers test the soil carefully before they apply fertilizers.

The use of herbicides varies with community and need. The use of 2,4-D to control broadleaf annual weeds in corn is common. The pre-emergence application of several newer chemicals controls both broadleaf and grassy annual weeds.

Weedkillers must be used with caution on grain sorghum for seed because the small, shallow-planted seedlings are susceptible to injury.

Many growers treat soil with insecticides as spray or granules by broadcast or row application against wireworms, rootworms, white grubs, seed corn maggot, seed corn beetle, cutworms, and others. Spray or granule applications to the growing plants are used to control the corn borer, corn earworm, corn leaf aphid, army-worm, sorghum webworm, grasshopper, and chinch bug.

Control of weeds in corn is important for maximum yields but is not important in obtaining weed-free seed.

Weed control in grain sorghum is essential in order to produce seed that is practically free of weed seeds and entirely free of seeds of noxious weeds. Sometimes a grower has to resort to hand operations to control weeds.

SEED FIELDS for corn usually are 40 rods from other corn of the same type and color. This distance sometimes is reduced when extra male rows are planted for added pollen production along the side of the field next to one or, which corn is being grown for feed.

Fields of different types of corn (like white and yellow or sweet corn and field corn) must be at least 80 rods apart. Seed fields of waxy or other special endosperm types and special plant types, such as dwarf corn, have to be at least 40 to 80 rods apart to avoid contamination.

All volunteer plants within the field and in fence rows or lots within the isolation distance must be removed before the seed parent flowers.

Early planting is preferred in order to gain early maturity in the fall and to have the corn at safe moisture levels in the event of an early freeze. Germination of grain with moisture levels of 35 to 40 percent may be reduced in a few hours by temperatures of 26 to 28 F. Germination of the grain at a moisture content of 30 percent and less may escape injury.

Early planting in the Corn Belt increases the need to use chemicals to control corn borers.

Differences in maturity may require different planting dates for the seed parents and the pollen parents so that the silk emerges at the same time the pollen is shed. Because the difference in planting dates may be 2 or 3 weeks, problems in tillage may arise. Control of weeds by chemicals may be especially helpful in such instances.

Heat units are often used as an aid in estimating when to plant the delayed seed parent. A heat unit is the number of degrees the average daily temperature exceeds a base usually 50 F. for corn near the low temperature at which growth occurs. Thus, with an average temperature for the day of 62 , 12 heat units occur.

Records from special nursery plantings give comparable data as to the heat units to flowering and silking for the different seed parents when they are planted at the same time. Such records tell whether the two parents will nick at flowering time or, if not, how much the early parent must be delayed in date of planting. With a difference of 100 heat units, it is necessary to delay planting the early parent until 100 heat units have occurred.

The system is not infallible. In seasons when May temperatures are well above normal, the delay may require 10 to 20 percent more heat units than normal. Similarly, in cool seasons, the number of heat units required will be fewer than normal.

Seed fields usually are planted in alternating strips of two rows of the pollen parent and six rows of the seed parent. Extra pollen rows are planted on the sides of fields or across the ends, as may be needed for mechanical convenience (to make it easier to use machinery on odd-shaped fields, for example) or for border rows as protection against nearby corn.

DETASSELING of corn is the removal of the tassel from each plant of the female seed parent at the proper stage of growth but before pollen is produced. The tassel on an individual plant may be pulled during a period of 6 to 18 hours. Because plants differ in time of tasseling, however, the time in which a field is detasseled may be from 5 to 10 days.

Crews must patrol each row four to seven times during the season and must go over the fields every 24 to 48 hours. The workers walk or ride through the field. They remove the tassel by a gentle upward pull and drop it to the ground. This procedure does not reduce the grain yield very much.

Detasseling crews often include girls, women, and boys who are available for part-time summer work. Adults are sometimes hired on a contract arrangement that allows them to de-tassel on a piecework basis. They get higher hourly wages than the younger workers.

Standards for detasseling vary somewhat by producer and by State certification agencies. In general, detasseling must be done so that not more than i percent of the tassels are shedding pollen in any one day. The total cumulative shedding for any three inspections must not exceed 2 percent. These tolerances are met without difficulty, except in bad weather.